A central paradigm to DNA damage response is the accumulation of the tumor suppressor p53 and a p53-mediated cell cycle response. Thus, p53 has been proposed to be a "sensor" to trigger an appropriate response in DNA-damaged cells. p53 accumulation mediates either a growth arrest at the G1/S boundary, or apoptotic cell death and is proposed to prevent the proliferation of malignant cells. p53 undergoes postranslational modifications, including phosphorylation and acetylation. DNA damage causes de novo phosphorylation in the amino-terminal region of p53. However, the mechanism by which p53 is stabilized and activated has not been established. The purpose of this research proposal is to test the hypothesis that DNA damage- induced phosphorylation is mechanistically significant for p53 function in vivo. Thus, in order to test the significance of amino-terminal phosphorylation and how it relates to p53 stability and function in a physiological context, I propose to generate mice and cells that contained an altered site of DNA- damage-induced phosphorylation. I will analyze these mice to determine the contribution of phosphate at Serine 18 in the function of p53 in DNA damage-induced G1 arrest, cell growth, and tumor suppression.